73K224BL
V.22bis, V.22, V.21, Bell 212A, 103
Single-Chip Modem w/ Integrated Hybrid
DATA SHEET
DESIGN CONSIDERATIONS
APPLICATIONS INFORMATION (continued)
Semiconductor's one-chip modem products
include all basic modem functions. This makes
these devices adaptable for use in a variety of
applications, and as easy to control as
conventional digital bus peripherals.
DIRECT ACCESS ARRANGEMENT (DAA)
The DAA (Direct Access Arrangement) required for the
73K224BL consists of an impedance matching
resistor, telecom coupling transformer, and ring
detection and fault protection circuitry.
Unlike digital logic circuitry, modem designs must
properly contend with precise frequency
tolerances and very low level analog signals, to
ensure acceptable performance. Using good
analog circuit design practices will generally result
in a sound design. Following are additional
recommendations, which should be taken into
consideration when starting new designs.
The transformer specifications must comply with the
impedance of the country in which the modem is being
operated. Transformers designed specifically for use
with the telephone network should be used. These
may present a DC load to the network themselves (a
“wet” transformer) or they may require AC coupling
with a DC load provided by additional devices (a “dry”
transformer). A dry transformer will generally provide
higher performance and smaller size than a wet
transformer. A wet transformer allows a simpler
design, but must not saturate with the worst case DC
current passing through it or distortion and poor
performance will result.
CRYSTAL OSCILLATOR
The K-Series crystal oscillator requires a parallel
mode (anti-resonant) crystal, which operates at
11.0592 MHz. It is important that this frequency be
maintained to within ±0.01% accuracy.
The protection circuitry typically consists of a transient
suppression device and current limiter to protect the
user and the telephone network from hazardous
voltages that can be present under fault conditions.
The transient suppresser may be a MOV (metal oxide
In order for a parallel mode crystal to operate
correctly and to specification, it must have a
capacitor connected to the junction of each of the
crystal and internal inverter connections,
terminated to ground. The values of these
capacitors depend primarily on the crystal’s
characteristics, and to a lesser degree on the
internal inverter circuit. The values used affect the
accuracy and start up characteristics of the
oscillator.
®
varistor), Sidactor (Teccor Electronics Inc.), spark
gap device, or avalanche diode. Some devices clamp
the transient to their specified break down voltage and
others go into low impedance crowbar state. The latter
require that the fault current cease before they can
return to their inactive state.
LAYOUT CONSIDERATIONS
Current limiting devices can consist of a resistor,
®
Raychem PolySwitch resettable fuse, or slow blow
Good analog/digital design rules must be used to
control system noise in order to obtain highest
performance in modem designs. The more digital
circuitry present on the PC board, the more this
attention to noise control is needed. The modem
should be treated as a high performance analog
device. A 22 µF electrolytic capacitor in parallel
with a 0.1 µF ceramic capacitor between VDD and
GND is recommended. Liberal use of ground
planes and larger traces on power and ground are
also highly favored. High speed digital circuits tend
to generate a significant amount of EMI (Electro-
Magnetic Interference), which must be minimized
in order to meet regulatory agency limitations. To
accomplish this, high speed digital devices should
be locally bypassed, and the telephone line
interface and K-Series device should be located
close to each other near the area of the board
where the phone line connection is accessed. To
fuse that can withstand the transient tests without
permanent damage or replacement.
Ring detection circuitry is not required by the FCC, but
may be required by the application. The ring detector
usually consists of an optoisolator, capacitor, and
resistor to present the proper AC load to the network
to meet the REN (Ring Equivalency Number)
regulations of FCC Part 68. The K-Series Design
Manual contains detailed information on the design of
a ring detect circuits as well as the other topics
concerning the DAA.
Page: 29 of 33
© 2005, 2008 TERIDIAN Semiconductor Corporation
Rev 7.1
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